Heat exchanger for liquid and gaseous media

ABSTRACT

A tube bundle heat exchanger has heat exchanger tubes which are formed over the length of the tube and especially the central region with uniform constricting formations. The degree of constriction, however, is reduced from the central region toward the outlet end, e.g. by reducing the depth of the formations, by increasing the pitch or pitch angle of helical formations, by increasing the spacing of the formations or by any combination thereof. The result is enhanced heat exchange efficiency and/or reduced pressure drop.

FIELD OF THE INVENTION

[0001] The present invention relates to a heat exchanger for liquid and gaseous media and especially to tube bundle heat exchangers and cooling of the exhaust gases of engines.

BACKGROUND OF THE INVENTION

[0002] Tube bundle heat exchangers which can be used for liquid and gas heat exchangers generally and as coolers for the combustion or exhaust gases of internal combustion engines generally comprise tube sheets supporting the tube bundle and located at inlet and outlet ends of the tube bundle and tubes which effect the heat exchange across the walls thereof and which have between the inlet and outlet regions of each tube a central region.

[0003] There are various heat exchanger tube constructions which provide in this intermediate or central region of the tube repetitive structures which partially constrict the flow cross section of the respective tube. These structures can be grooves, embossed projections, crescents and like configurations which can extend with a certain pitch, to the extent that they are helical, or can be shaped to impart a twist to the fluid flowing through the tube.

[0004] The structures contribute to the heat exchange and may promote turbulence within the tube.

[0005] A heat exchanger tube of this type is described in the European Patent EP 0 772 017 A2.

[0006] That heat exchanger tube is especially designed for a tube bundle heat exchanger of a heat recovery boiler and has circular end cross sections with depressed wall structures along the intermediate region between the end portions. The intermediate region between the circular cross section portions is flattened to provide a slender flow passage whose cross section, however, is matched to the flow requirements of the hot gases traversing that tube.

[0007] In that document and other publications describing heat exchanger tubes in which formations project into the flow path of the fluid within the tube to form the partial constrictions, there have been compromises with respect to the heat transfer efficiency for pressure drop. Nevertheless we have found that an improvement in the ratio of heat transfer efficiency to pressure drop is possible.

OBJECTS OF THE INVENTION

[0008] It is the principal object of the present invention, therefore, to provide an improved heat exchanger tube for a tube bundle heat exchanger for liquids or gases and especially a tube bundle heat exchanger for the cooling of the exhaust gases of a combustion engine, which has an enhanced ratio of heat exchange efficiency to pressure drop by comparison with earlier systems.

[0009] Another object of this invention is to provide a heat exchanger tube for a tube bundle heat exchanger with improved heat transfer efficiency and reduced pressure drop.

[0010] It is also an object of this invention to provide a tube bundle heat exchanger of improved efficiency and reduced pressure drop.

SUMMARY OF THE INVENTION

[0011] These objects and others which will become apparent hereinafter are attained, in accordance with the invention in a tube bundle heat exchanger tube which is provided at an intermediate location along its length between the inlet and outlet portions of that tube with formations or structures projecting inwardly and partially constructing the flow cross section of the tube and wherein at least in a portion of the length of the tube, the number of such formations per unit length or the flow cross section constricting volume of such formations per unit length or the degree of flow constriction per unit length is reduced toward the outlet end of that tube.

[0012] The formations or structures can be helical grooves or ribs or helical or angular corrugations or other formations extending over at least half the circumference or periphery of the tube.

[0013] Where helical grooves or ribs are provided, the pitch (d₂) or pitch angle at the outlet portion of the tube can be greater than the pitch angle or pitch (d₁) at the intermediate portion.

[0014] The result of the reduction in the degree of constriction per unit length toward the end is a reduction in the pressure drop without a corresponding reduction in the heat transfer efficiency and, therefore, a greater heat transfer efficiency for a given or tolerable pressure drop for the heat exchanger tube as a whole.

[0015] The formations, which operate at least in part by promoting turbulence over the length of the heat exchanger tube, appear to have a reduced effect toward the outlet end of the tube and thus the reduction in the degree of constriction per unit length toward the outlet end of the heat exchanger does not significantly affect the heat transfer.

[0016] The reduction in the degree of constriction per unit length can be effected by, for example, increasing the pitch or pitch angle form the intermediate region toward the outlet region, by increasing the spacing between the formations toward the outlet region, by decreasing the depth of the formations toward the outlet region and by increasing the spacing between the formations toward the outlet region.

[0017] The effect desired with the present invention can also be achieved by increasing the diameter or cross sectional area of the tube toward the outlet end. By increasing the cross section area of the tube toward the outlet end of the tube, the flow velocity within the tube can be reduced and that can contribute to an increase in the static pressure which results in an overall lowering of the pressure drop in the tube.

BRIEF DESCRIPTION OF THE DRAWING

[0018] The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

[0019]FIG. 1 is a side elevational view, partly broken away, of a heat exchanger provided with heat exchanger tubes according to the invention;

[0020]FIG. 2 is a simplified cross section through the tube bundle of the heat exchanger of FIG. 1;

[0021]FIG. 3 is a side elevational view of an axial section at the outlet end region of a portion of a heat exchanger tube having helical grooves;

[0022]FIG. 4 is a view similar to FIG. 3 of another heat exchanger tube with helical grooves formed therein;

[0023]FIG. 5 is a side elevational view of a heat exchanger tube, seen in section at the outlet end, in which the formations are corrugations;

[0024]FIG. 6 is an elevational view of a heat exchanger tube according to the invention shown in section upstream of the outlet end and illustrating another embodiment of the invention in which the tube widens to the outlet end;

[0025]FIG. 7 is a view similar to FIG. 6 wherein the formations are corrugations rather than helical grooves;

[0026]FIG. 8 is an elevational view of a heat exchanger tube with a flattened central portion between the inlet and outlet portions and in which the formations are indentations formed in opposite sides of the tube;

[0027]FIG. 9 is a view of the tube of FIG. 8 in the direction of arrow IX in FIG. 8;

[0028]FIG. 10 is a side view of another heat exchanger tube according to the invention; and

[0029]FIG. 11 is a view in the direction of arrow XI of the tube in FIG. 10.

SPECIFIC DESCRIPTION

[0030] In the drawing, we have shown a tube bundle heat exchanger 10 which comprises a bundle 10 a of heat exchanger tubes 11 mounted in tube sheets, one of which is shown at 10 b in FIG. 1. The heat exchanger body is formed by a housing 12 with two end caps 10 c and 10 d formed with an inlet opening 13 and an outlet opening 14 for the gas which is to traverse the tubes 11 and to be cooled. The inlet and outlet openings 13 and 14 are fittings connectable in a line traversed by the gas to be cooled, e.g. exhaust gas of an internal combustion engine. Opening at opposite ends and diametrically opposite one another into the casing 12 are inlet and outlet fittings 15 and 16, respectively, for a cooling medium, e.g. water, which flows around the tubes 11 through the housing.

[0031] As can be seen from FIG. 2, the tubes 11 are arranged in a close packed but spaced relationship.

[0032]FIG. 3 shows one of the heat exchanger tubes 11 a which can have an inlet region 17, a central region 18 and an outlet region 19. The heat exchanger tube 11 a is formed in its middle region 18 with a uniform repetitive helical groove formation 20 with a constant pitch and constant pitch angle d₁. In the transition from the middle region 18 to the outlet region 19, however, there is a variation in the helical grooves 20 which is effectively an increase in the pitch angle d₂ or the pitch. With this change in the pitch angle d₁/d₂, there is an increase in the spacing of the structures which partially constrict the interior of the tube. This can best be seen at the right hand side of FIG. 3 which is in section to the right of the section plane x₁.

[0033] There it is evident that the helical grooves 20 result in an inwardly projecting formation 20′ which partially constricts the flow cross section.

[0034]FIG. 4 shows another heat exchanger tube 11 b which differs from the heat exchanger tube 11 a only in that there is no change in the pitch angle d₁/d₂ from the middle region 18 to the outlet region 19 (shown in section to the right of the section plane x₂) but rather there is a change in the depth t₁ in the central region to t₂ outlet region. The depths of the grooves 20 are reduced toward the outlet region 19 so that the amount of constriction of the tube is less in the outlet region. The reduction in the groove depth t₁/t₂ corresponds to the magnitude of the flow cross section constriction in the outlet region 19.

[0035]FIG. 5 shows a heat exchanger tube 11 c wherein, instead of helical grooves forming the flow construction of the heat exchanger tube or the turbulence performing formations, the latter are constituted by numerous corrugations 21. In the outlet region there is an increase in the spacing s₂ of the corrugations from the spacings in the central region 18. In this FIG. as well a section plane x₃ is provided to illustrate a cross section through the tube to the right of that plane. In an analogous manner, the corrugation depth within the tube can be reduced to reduce the degree of constriction per unit length in the outlet region.

[0036] The heat exchanger tube lid of FIG. 6 has helical grooves 20 whose pitch angle d and depth t into the flow cross section do not change along the length of the tube but wherein the flow cross section of the tube is increased by the formation of a funnel which widens outwardly at the outlet end 19. This increase in the flow cross section results in a reduction of the kinetic energy of the flow at the outlet region and at an increase in the static pressure so that the pressure drop over the entire tube length is reduced by comparison with a tube of uniform flow cross section.

[0037] A similar construction is found in FIG. 7 in which the heat exchanger tube lle has numerous annular corrugations 21 with a constant passing and a constant depth. Here again, the funnel shaped outlet end of the tube results in a reduction of the degree of constriction of the tube toward this outlet end.

[0038]FIGS. 8 and 9 show a heat exchanger tube 11 f which is flattened in the central portion 18 and in which the opposite services or sides have indentations 22 which within the tube constitute constricting formations. The spacing of these formations is greater toward the outlet end 19 of the tube then at the central region 18. FIG. 10 and 11 show another flattened configuration of a heat exchanger tube 11 g and wherein indentations 23 of a drop at shape are provided in the opposite walls.

[0039] These indentations have a greater spacing or reduced density at the outlet regions. The small indentations have the advantage that they generate turbulent boundary layer flow with a minimum pressure drop. The heat exchanger tubes of the invention can be fabricated by any shaping process which is appropriate for the material used and the particular metal or metal alloy. The tubes may be formed by any tube rolling process or preformed tubes may be provided with the formations by rolling, stamping or pressing operations and, especially, by hydroforming techniques. Rolling between disks to shape the tubes can also be used. 

We claim:
 1. A heat exchanger tube for a tube-bundle heat exchanger, said tube having an inlet end, an outlet end and a central portion extending between said ends, said tube being provided with substantially regular formations over said central portion partially projecting into a path of flow of a fluid medium traversing said tube from said inlet end to said outlet end, said formations being configured to reduce a degree of constriction of a flow cross section of said tube toward said outlet end.
 2. The heat exchanger tube defined in claim 1 wherein a density of said formations per unit length diminishes over an outlet portion of said tube toward said outlet end thereof.
 3. The heat exchanger tube defined in claim 2 wherein a number of said formations increases over said outlet portion of said tube toward said outlet end.
 4. The heat exchanger tube defined in claim 2 wherein a spacing of said formations increases over said outlet portion of said tube toward said outlet end.
 5. The heat exchanger tube defined in claim 2 wherein said formations are in the form of helical grooves and a pitch or pitch angle of said grooves increases over said outlet portion toward said outlet end.
 6. The heat exchanger tube defined in claim 2 wherein said formations are corrugations in said tube and a spacing of said corrugations increases over said outlet portion toward said outlet end.
 7. The heat exchanger tube defined in claim 2 wherein said formations are in the form of helical grooves and a depth of said grooves decreases over said outlet portion toward said outlet end.
 8. The heat exchanger tube defined in claim 2 wherein said formations are corrugations in said tube and a depth of said corrugations decreases over said outlet portion toward said outlet end.
 9. The heat exchanger tube defined in claim 1 wherein said formations are spaced-apart indentations in a wall of said tube and a spacing of said indentations increases over an outlet portion of said tube toward said outlet end.
 10. The heat exchanger tube defined in claim 1 wherein said formations are spaced-apart indentations in a wall of said tube and a number of said indentations increases per unit area decreases over an outlet portion of said tube toward said outlet end.
 11. The heat exchanger tube defined in claim 1 wherein said formations are spaced-apart indentations in a wall of said tube and a depth of said indentations decreases over an outlet portion of said tube toward said outlet end.
 12. The heat exchanger tube defined in claim 1 wherein said formations are spaced-apart indentations in a wall of said tube and arranged transversely of a flow direction of said fluid medium through said tube.
 13. The heat exchanger tube defined in claim 1 wherein said formations are spaced-apart indentations in a wall of said tube and arranged in a flow direction of said fluid medium through said tube.
 14. The heat exchanger tube defined in claim 1 wherein said ends are of circular cross section and said central portion has a flattened cross section.
 15. The heat exchanger tube defined in claim 1 wherein said tube has a flow cross section wider at said outlet end that at said central portion.
 16. The heat exchanger tube defined in claim 15 wherein said tube widens continuously toward said outlet end.
 17. The heat exchanger tube defined in claim 16 wherein said tube has a funnel-shaped outlet portion provided with the reduced reduce degree of constriction of said flow cross section of said tube toward said outlet end.
 18. The heat exchanger tube defined in claim 1 wherein said central portion has an oval cross section widening into said inlet and outlet ends.
 19. A heat exchanger comprising: a housing; a tube bundle in said housing having a multiplicity of heat exchanger tubes mounted in tube sheets; an inlet for a fluid medium communicating with inlet ends of said tubes; an outlet for said fluid medium communicating with outlet ends of said tubes; and an inlet and an outlet fitting connected to said housing for passing another medium around said tubes, each of said tubes having a central portion extending between said ends, said tubes each being provided with substantially regular formations over said central portion partially projecting into a path of flow of a medium traversing said tube from said inlet end to said outlet end, said formations being configured to reduce a degree of constriction of a flow cross section of said tube toward said outlet end.
 20. The heat exchanger defined in claim 19 wherein a density of said formations per unit length diminishes over an outlet portion of each of said tubes toward said outlet end thereof. 